widlar transistors chapter 1

7/25/2019 Widlar Transistors Chapter 1

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DEPARTMEM

OF

WFAPONS

TRAINIM

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BASE

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GOLCRADO

TRANSISTORS

IN-SERVICE

TRAINING

COURSE

INTRODUCTION

TO

SEMICONDUCTOR

DEVICES

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2/17

1

I

CHAPTER

1

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A/10

Robert

J.

Widlar

1;

Abstract

The

idea

of

conduction

in

solids

by

both

holes

and

electrons

is

introduced.

The

estence

of

two

distinct

types

of

current

carriers

is

demonstrated

using

the

Hall

effect.

Without

explaining

the

origin

of

these

current

carriers

an

elementary

description

of

junction

diodes

and

two jmction

transistors

is

given.

Finally

it

is

shown

how

a transistor

can

be

used

in

an

amplifier

circuit.

i


3/17

Q

5

INTRODUCTION

It

is

difficult

to

find

a.

piece

of

modern

electronic

equipment

that

does

not

incorporate

some

kind

of

semiconductor

device,

such

as;

a

selenium

rectifier,

f

almost

al.l

commercial,

industrial,

a

crystal

diode,

or

a

transistor.

This

is

true

o

b

a

these

semiconductor

devices

are

generally

smaller,

and

military

equipment

ec

use

more

efficient

and

in

some

cases,

more

reliable

than

components

used

in

the

past.

The

existence

of

semiconductors

has

been

known

for

many

years;

but,

until

recently,

they

were

considered

a

useless

transition

between

conductors

and

insula-

tors.

They

first

enjoyed

widespread

use

in

the

early

days

of

radio

when

it

was

discovered

that

the

contact

between

a

fine

wire

and

some

substances

-

such

as

alina

iron

pyrite,

and

silicon

carbide

-

exhibited

rectifying

properties.

The

tubes

pushed

semiconductors

E

0

.

reason

for

this

effect

was

not

known.

Therefore,

vacuum

duction

in

a

vacuum

was

well

understood;

and

an

into

the

background,

because

con

intelligent

approach

could

be

used

in

the

design

and

perfection

of

tubes.

The

influx

of

radar

and

other

complex

electronic

systems

during

World

liar

II

spurred

the

development

of

semiconductor

diodes;

but,

because

of

the

pressing

need,

d

This

effort

resulted

in

practical

silicon

and

an

experimental

approach

was

use

.

selenium

rectifiers

;'

however,

it

did

not

provide

_a

major

breakthrough,

as

the

phenomenon

of

conduction

in

solids

was

still:

not

completely,

nderstood.

The

major

t

ansistor

a

semiconductor

.

advance

came

in

19148

with

the

development

of

the

r

,

It

ared

that

the

transistor

could

perform

the

same

function

amplifying

device.

apps

as

a

vacuum

tube,

while

using

one

hundredth

the

power

and

occupying

far

less

ace

This

added

impetus

to

semiconductor

research,

and

workable

theories

on

SP

the

operation

of

semiconductors

were

evolved.

the

research

on

transistors

was

applied

to

other

The

knowledge

gained

from

devices,

md

soon

silicon

and

germanium

diodes

were

made

with

ratings

exceeding

those

of

the

best

copper

oxide

and

selenium

rectifiers.

Also,

the

silicon

solar

f

the

sun

directly

into

cell

was

developed

which

could

convert

the

energy

o

abl

efficiency

Diode

amplifiers

for

example,

the

ab

electricity

with

reason

e

.

varactor

and

the

tunnel

diode

were

made

which

could

perform

at

frequencies

ove

the

range

of

transistors.

These

and

many

other

devices

benefited

from

semi-

conductor

research.

t

t

osition

in

the

It

can

be

seen

that

semiconductors

have

assumed

an

impor

an

p

used

with

increasing

frequency

because

some

electronics

industry.

semiconductori

device

might

be

able

to

replace

a

whole

ci

onnected

with

electronics

to

components.

It

is

therefore

necessary

for

anyone

become

familiar

with

semiconductor

theory

just

as

he

was

required

to

learn

the

.

theory

of

vacuum

tubes.

One

of

the

most

important

fundamentals

in

the

theory

of

semiconductors

is

'

'

rd

to

understand

this

phenomenon

the

mechanism

of

conduction

in

solids.

But

in

o

er

the

structure

of

solids.

The

chemistry

and

physics

it

is

also

necessary

to

learn

of

a

few

important

semiconductor

materials

will

be

discussed

in

chapter

two.

1-1


4/17

9

0

-

a

Furthermore,

the

characteristics

of

these

substances

important

to

the

understanding

of

practical

semiconductor

devices

will

be

brought

forth.

In

later

chapters,

the operation

of

transistors

and

othr

PN

junction

devices

will

be

given

a detailed

explanation.

Emphasis

will

be

placed

on

the

junction

diode

and

the

two-junction

transistor

because

these

will

illustrate

the

more

significant

results

that

can

be

realized

with

semiconductors.

Still

later,

the

techniques

used

in

the manufacture

of

these

devices

will

be

introduced,

and

most

of

the

processes

currently

used

in

the

production

of

diodes

and

transistors

will

be

briefly described.

The

last

chapter

will

be

devoted

primarily

to

the

circuit

applications

of

transistors

although

other

devices

of

current

interest

will

be

mentioned.

Before

going

into

a

more

exact

explanation

of

semiconductor

phenomenon,

a

beief preview

of

conduction

in

solids

will

be

given

and

then

related

to the

operation

of

the

junction

diode

and

the

tranistor.

The

purpose

cfthis is

to give

the

reader

an

opportunity

tdgget

a

overall

view

of

the

material

to

be

covered. _Exact

explanations

and

detailed

proofs

will

not

be

used

here.

CONUGTION

IN

SEMCONDUTORS

T

.

Semiconductors

are

a

class

of

materials

having

conductivities

somewhere.

between

those

of

metalic

conductors

and

insulators.

The

conductivity

of

these

materials

is

several

orders

of

magnitude

greater

than

insulators

but

still

very

much

less

than

metalic

conductors.

A

simplified

picture

of

conduction

in

metals

attributes

conduction

to

the

existence

of

free

electrons

which

can

be

moved

under

the

influence

of

an

electric

field.

It

then

seems

reasonable

to

assume

that

free

electrons

are

also

responsible

for

conduction

in

semiconductors.

This

is

the

case

for

some

semiconductor

materials,

but

it

has

been

shown

that

conduction

can

also

take

place

by

what

appears

to

be

a.positive

electron,

or

hole.

The

properties

of

semiconductors

of

concern

at

this

point

are

that

the

current

carriers

can

be

either

holes

or

electrons

and

that

the

concentration

and

type

of

current

carrier

can

be

controlled

during

production

of

the

material.

This

much

will

be

assumed

here,

but

to

help

substantiate

these

statements

a

demonstration

of

the

existence

of

holes

follows:

T

It

was

found

that

the

current

carriers

in

semiconductors

traveled

at

greater

velocities

than

those

in

metals.

~It

was

thought,

then,

that

these

current

carriers

could

be

deflected

apreciably

from

their

normal

path

within

the

semiconductor

by

the

application

of

a

magnetic

field.

This

was

indeed

the

case

as is

shown

in

Figure

1.1.

A

current

was

passed

between

two

metalic

contacts

on

a

block

of

semiconductor

material.

Two

other

electrodes

were

placed

at

right

angles

to

the

current

flow

as

can

be

seen

from

the

figure.

If

the

current

carriers

were

electrons,

the

results

shown

in

Figure

l.la.

could

be

expected;

with

no

magnetic

field

applied,

the

number

of

electrons

reaching

the

two

electrodes,

at

right

angles

to the

current

flow

would

be

equal,

and

there

would

be

no

potential

difference

between

these

electrodes.

When

a

magnetic

field

is

applied

into

the

page

as

shown,

the

electrons

would

be

\

deflected

upward;

and

moreeelectrons

would

reach

the

uper

contact

than

would

reach

the

lower.

The

upper

contact

would

then

become

more

negative

than

the

lower,

and

a

potential

difference

couldlne

measured

between

them.

This

was

found

to

be

the

case

for

some

semiconductor

materials.

However,

with

other

materials,

the

opposite

3;

effect

was

observed:

when

the

magnetic

field

was

applied

in

the

same

direction,

relative

to

current

flow,

the

upper

contact

became

positive

with

respect

to

the

lower.

This

could

not

happen

with

electron

current

carriers.

It

was

then

assumed

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FIELD

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,

that

there

were

two

distinct

types

of

current

carriers in

semiconductors,

electrons

and

holes.

This

would

explain

the

effect

observed

in

Figure

l.lb,

This

is

known

as

the

Hall

effect,

and

in

practice

it

is used

to

measure

steady

magnetic

fields

for

the detection

of

submarines;

for tape

recorder

heads;

and

in

circulators,

gyrators,

and

isolators

where

the

direction

of

current

flow

is

altered

by

a

magnetic

field.

when

the

current

carriers

in a

semiconductor

are

free

electrons Figure

l.2a)

conduction

is

relatively

easy

to

visualize.

When

a

voltage is aplied

across

the

semiconductor,

the

electrons

will

move

from

negative

to

positive

and

into

the

external

circuit.

lwhen

an

electron

passes

out

of

the

semiconductor

into

the

external

circuit

at

the

positive

terminal,

another

electron

immediately

flows

into

the

semiconductor

at

the

negative

terminal.

V-__

A

When

holes

carry

the

current

in

a

semiconductor,

there

must

be

a transition

at the

circuit

connections.

This

is because

the

current

carriers

in

the

external

circuit

are

electrons

while

the

current

carriers

in

the

semiconductor

are

holes.

If

an

electric

field

is

applied,

the

holes

will

flow

from

positive

to

negative

until

they

reach

the

negative

terminal

as

shown

in

Figure

l.2b.

-When

a

hole

does

reach

the

negative

terminal,

it

catures

a

free

electron

from

th

external

circuit

and

the

hole

disappears.

At

the

same

time

the

external

circuit

recovers

an

-

electron

from

the

positive

terminal,

creating

another

hole.

_This

new

hole

will

travel

to the

negative

terminal

repeating

the

process.

By

this

mechanism,

current

is

carried

by

holes

within

the

semiconductor,

and

by

electrons

in

the

metalic

conductors

of

the

external

circuit.

1

Some

helpful

rules

for

predicting

conduction

in

semiconductors

will

be

stated

here

without

proof.

Whenever

an

electron

is

removed

from

a semiconductor

under

the

influence

o

an

apptied

electric

field,

it

must

soon

be

replaced

by

an

electron

from

the

potential

source

external

circuit)

to

keep

the

overall nnber

of

electrons

constant.

Similarly,

whenever

a

hole

is

removed

from

a

semiconductor

by

recombination

with

an

external

electron,

the

hole

must

be

replaced

by

the

removal

of

another

electron

to

keep

the

overall

number

of

holes

constant.

To

appreciably

change

the

nuber

of

holes

or

electrons

would

require

high

electric

potentials

which

are

never

encountered

in

practice.

Another

detail

concerns

the

production

of

holes

by

the

removal

of

electrons:

This

can

only

occur

to

any

appreciable

extent

at the

terminals

contacts)

of

the

semiconductor.

Hole

genera-

tion

occurs

at

imperfections

in

the

physical

structure,

ad

there

are

normally

very

few

imperfections

within

the

semiconductor.

However,

the

surface

conditions

resulting

from

attaching

a

contact

create

many

imperfections.

Hole

generation

can

therefore

take

place

far

more

readily

at

the

contacts

than

it

can

within

the

semiconductor.

@

JUNCTION

DIODES

It

can

be

seen.then,

that

there

are

two

distinct

classes

of

semiconductors:

N

type

in

which

the

current

carriers

are

free

electrons,

and

P

type

in

which

the

current

carriers

are

equivalent

to

positive

electrons

holes).

This

fact

can

be_

used

to

build

many

useful

devices,

one

of

which

is

the

junction

diode.

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8/17

o

i A

junction

diode is

made

by

somehow

joining

together

P

and

N

type

semiconductor

materials.

A diagram

of

a

junction

diode

is

shown

in

Figure 1.3,_

Contacts

are

made to

both the

P

and

N

type

materials

so

that the

device

can

be

connected

to

an

, vexternal circuit.

A device

constructed

in

this

manner

will

exhibit a low

resistance

to

current

flow

in

one

direction

and

a

high resistance

in

the

other,

.A

mechanical

s

view

of

this

action

follows:

V

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17/17

v

Although

the

listed

power

output of 375

mar may

seem

low, it must

be

remembered that this

is

about

the same

available from

a

large

battery-powered

portable

radio,

.

V

a

This

single

circuit

obviously

does-not

begin

to describe

the

pgssibities

of.

the

transistor,

Amplifiers and

oscillators

can

be

made

that

H111

operate

'

with

a

d-c input

power

as low

as

afew

microwatts-or,

using

power

transistors,

with outputs

in

the

order of several

hundred

watt,

In addition,

efficiencies

approaching

theoretical

values

can be

attained

with

practical

circuits

in

switching and

audio

applications.

Furthermore,

there

are

NPN

transistors

which

operate

in

the

same

manner

as

PNP

units

except

that

the role

of

the

holes

and

electrons

are

interchanged.

The

NPR

transistors

function

with

voltages of

opposite

polarity

wes.

This

*permtsd uts

that

*

would

not

be possible

with

vacuum

tubes,

as

this

corresponds

to having a tube

that

will

operate

with

a

negative

plate voltage. u

However,

transistors do

have

many

limitations

that have

not yet

been

mentioned,

but

these

limitations can

be

overcome

with

proper circuit

design,

Therefore,

- any

serious

discussion of

circuits using

semiconductor diodes,

'

and

amplifiers

will

be

put

off

until a

better

understanding

of

the factors

~

affecting

semiconductor performance,

mainly

temperature

and

operating

frequency

is

gained,

I

l-15

widlar transistors chapter 1

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